This invention relates to novel and efficient methods of producing steels and steel products having relatively high carbon or manganese content, or both, with good surface quality, being particularly the surface characteristics of rimmed steel, and to certain steel products so made. More particularly, the invention is concerned with the manufacture of steels and steel products with which to achieve desirable properties in the area of strength, toughness, ductility, magnetic characteristics or the like, where the required proportion of manganese or carbon, or both elements, is significantly higher than would normally be contemplated in order to permit good rimming action in the melt of steel employed.
A chief reason for wanting a steel product which is of properly rimmed character is that such products have excellent surface quality, as well as certain ancillary benefits usually associated with the surface regions of rimmed steels. As indicated, however, there are definite limits to the content of carbon or manganese (or both) that can be employed without adversely affecting the rimming action and without likewise impairing the good surface quality achieved by such action. Thus, for example, higher carbon steels, as up in the range of 0.2 and greater percent, are particularly suitable for many structural purposes, as sheets or bars in hot rolled as well as cold rolled state. Yet such products cannot be made satisfactorily as rimmed steel because of the high carbon content. Hence, it has been impossible to obtain in a convenient, economical manner, the good surface condition of a rimmed product, at the indicated carbon levels. Likewise with or without the higher range of carbon, the manganese content of rimmed steels has necessarily been limited. Although compositions containing manganese above, and indeed well above 0.6%, are particularly suitable for commercial stamping and drawing grades (e.g. cold rolled products), such grades cannot be made as rimmed steels, or more particularly, cannot achieve the highly desirable surface characteristics of rimmed steel.
The prior art contains disclosures of methods wherein a melt of rimming steel, e.g. basically suitable for deep drawing purposes when cold rolled, has been poured to nearly fill an ingot mold, whereupon teeming has been interrupted for several minutes to permit rimming action while a skin of rimmed steel solidifies against the mold wall, and then teeming is continued to fill the mold with aluminum introduced into the falling stream of molten steel so that the ultimate ingot consists of a core of aluminum killed steel having a skin of rimmed steel. From such ingot, rolled products are described as made having basically the properties (in the core) of killed steel, with a rimmed surface; some suggestion has been made to add other elements, such as columbium, in very limited quantity for special purpose along with the aluminum, but there was essentially no thought of producing anything beyond the aluminum-killed, deep drawing product having a surface zone of rimmed steel.
Some more recent inventions, covered by pending patent applications, have embraced products having rimmed steel surface layers over cores that may not only be aluminum killed, but contain other special ingredients such as microalloying elements (e.g. Cb, V, Ti) providing a high strength, low alloy core of very low carbon content, or elements such as phosphorus for particular purposes; or alternatively rimming steel compositions, similarly providing a rimmed skin, with a rimmed core having a content of vanadium or boron such that the aging normally characteristic of rimmed steels if successfully retarded, without producing the difficulties that ordinarily accompany the use of the last-mentioned elements to make a rimmed steel non-aging.
None of the above disclosures, however, has been directed to steels, especially of structural grades, that basically require substantially higher levels of carbon or manganese, or both, and that therefore cannot ordinarily be produced as rimmed steel. In U.S. Pat. No. 2,389,516, Kinnear, Nov. 20, 1945, there is proposed a process where an ingot mold, e.g. of 19 to 20 tons capacity, is first poured to a level of about 65% with rimming steel such as 0.08% C and 0.38% Mn content, which is then allowed to rim or effervesce in its normal manner for about 15 minutes. Thereupon molten, killed or deoxidized steel, i.e. containing 0.78% C, 0.8% Mn and 0.26% Si, is added to the ingot mold on top of the first-introduced, rimming steel (of which the core is still molten), with the result that the additional steel diffuses into the central portion or core of the rimming steel, stopping the rimming action by the time the mold is entirely filled.
The solidifed result of the above patent, in the lower 65% of the mold, from which the upper 35% is cropped after rolling to slab, is described as a steel body consisting of a low-carbon skin and high-carbon core having higher strength and elasticity than the skin. This process, however, is relatively inefficient or impractical, not only because such a relatively large amount of the ingot is wasted but especially because two separate melts of steel must be prepared, and separately carried in ladles for the two pouring operations in each of the molds of a single drag. Aside from problems of making and timing the melts, the operation is cumbersome, and indeed beyond the capability of conventional melt shops, i.e. as to the requirement of providing and using, almost simultaneously, two separate cranes and two separate ladles. Hence, even as to steel of high carbon content, there has been no economical or truly feasible way of making a product with the clean qualities, at or near the surface, of rimming grades.